[time-nuts] Allan variance by sine-wave fitting

Bob kb8tq kb8tq at n1k.org
Mon Feb 26 14:51:47 EST 2018


Yet another reason to nuke the battery and the A2 board. 

It is amazing just how small a signal can mess things up at the levels involved in
a good frequency standard. The old “when in doubt, throw it out” mantra may be
a good one to keep in mind relative to a lot of add on features…. how much does
a PIC-DIV pull compared to the 1 pps section of one of these old beasts (Cs or Rb)? 

Lots to think about. 


> On Feb 26, 2018, at 2:20 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>> at telling you it was sitting on top of a power transformer. It didn’t matter a lot
> I did ten runs of various standards that were within a couple of meters of the bench; the TimePod did not move the entire time. Each standard had a different looking PN plot, so I'm pretty sure the 120 Hz spur we see is the 5065A itself, not something in the lab.
> File http://leapsecond.com/tmp/2018b-Ralph-2-pn.png is attached.
> Fun fact -- there's a wide spur at ~2 Hz on the 5065A phase noise plot. What do you think that is? On a hunch I opened the front panel and reset the blinking amber battery alarm lamp, and voila, that noise went away. Makes sense when you think of the power variations associated with a blinking incandescent lamp.
> /tvb
> ----- Original Message ----- 
> From: "Bob kb8tq" <kb8tq at n1k.org>
> To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Monday, February 26, 2018 7:00 AM
> Subject: Re: [time-nuts] Allan variance by sine-wave fitting
> Hi
> One of the TimePods that I had access to in the past was particularly good
> at telling you it was sitting on top of a power transformer. It didn’t matter a lot
> which instrument the power transformer was in. For some weird reason it
> was a good magnetometer at line frequencies. I never bothered to send it
> back for analysis. Simply moving it onto the bench top (rather than stacked 
> on top of this or that) would take care of the issue.
> As far as I could tell, it was just the one unit that had the issue. None of the
> others in the fleet of TimePods seemed to behave this way. Given that they
> normally are very good at rejecting all sorts of crud and ground loops, it was 
> somewhat odd to see. 
> Bob
>> On Feb 26, 2018, at 7:13 AM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>>> BTW: Do you know the cause of the oscillations in the 5065 vs BVA plot?
>> The ADEV wiggles aren't visible with normal tau 1 s measurements. But since the TimePod can go down to tau 1 ms, when I first measure a standard I like to run at that resolution so effects like this show up. Once that's done, 1 ms resolution is overkill.
>> In this case it appears to be power supply noise. Attached are the ADEV, PN, and TDEV plots.
>> The spur at 120 Hz is massive; there's also a bit at 240 Hz; almost nothing at 60 Hz. When integrated these cause the bumps you see in the ADEV plot. It's best seen as a bump at ~4 ms in the TDEV plot.
>> Note the cute little spur at 137 Hz. Not sure what causes the one at ~3630 Hz.
>> /tvb
>> <5065a-adev.png><5065a-pn.png><5065a-tdev.png>
> <2018b-Ralph-2-pn.png>_______________________________________________
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